Ignition advance curve

Hi,
I hope someone can help.
I have a 123 tune on my 1962, 3.8 mk2. It is standard apart from a classic fabs exhaust manifold and large bore system and, of course, the 123 tune. It is 8:1 compression, compression values of between 150 and 160 PSI.
I am trying to set up the advance curves and I am getting confused. The manual shows this -

Dizzy data.jpg

I have the 40640 dizzy and the test data in the workshop manual suggests the timing should be all in at 3400 revs and there should be around 19 degrees of advance (columns 8 and 9). The manual also says that static timing should be set to 7 degrees btdc, giving total advance of 26 degrees at 3400rpm.
Am I reading this correctly? Some other applications of the 3.8, 8:1 engine have timing of up to 38 degrees all in. This is a big difference.

So, I have set the centrifugal advance to this

centrifugal curve.jpg

but I have currently got the initial static timing set to 10 rather than 7, this can be accommodated (to a degree) in the 123 settings.

The original vacuum values are 8 - 18 - 7 according to the vacuum advance unit number (column 3), and in the dizzy data in the manual, the test data is in dizzy degrees not crank degrees (columns 4 and 5).

The original vacuum values are 8 - 18 - 7 according to the vacuum advance unit number (column 3), and in the dizzy data in the manual, the test data is in dizzy degrees not crank degrees (columns 4 and 5).

So I have set the vacuum advance to this

vacuum curve.jpg

If you convert the dizzy data for the centrifugal advance to crank degrees, you get all in at 3400 rpm of 2 x 19 + 7 = 45 degrees, far too much. Or is it? Add on 14 degrees vacuum and it is firing closer to bottom dead centre than top.

Do these settings look reasonable?

Thanks for your help.
Dave

I have been playing around and set the advance curve up like this -

It now has 35 degrees max advance coming in at 3500 revs.
It pulls much better now than with the earlier settings and revs higher too without getting asthmatic.

Trouble now is that any further adjustment is going to be minor and identifying any improvement likely to be subjective.

I remember being told that the advance should be ‘all in’ around 3300 rpm, so I may increase the advance between 2300 and 3300.

I think now the best thing to do is book a rolling road session.

Dave

1 Like

Hello
I equiped my 3,8l Stype with an 123 Electronic igniton, could you tell me the right curve to choose ?
Pierre Philippe Foucart
France

The various engines are a little different. It’s best to make a graph of the factory curve from the workshop manual. There are upper and lower limits presented in the two drawn curves. Find the 123 curve of the 16 curves I suspect that you have (not bluetooth?) and select the 123 curve from the instructions that fits the best inside the factory curves. The 123 will be more aggressive than factory up over 3500 to 4500 rpm on many of the curves. I rarely exceed 4000 to 4500 rpm in my cars. Paul.

The original factory curves are safe compromises on what could be achieved with springs and weights and a vacuum capsule working independently of each other. The curves in the 123 ought (I’ve no knowledge of them) to be a better overall shape. If the 123 curves are one or two degrees more advanced at some speeds, it should not put the engine at risk. The ultimate solution is to go to a rolling road to find the minimium advance for best torque and the knock limits as a function of engine speed and load.